Field of the Invention
The present disclosure relates to a pouch case for a secondary battery and a pouch type secondary battery including the same, and particularly in a pouch case including an inner layer, a metallic layer, and an outer layer for enhanced stability, to a pouch case for a secondary battery and a pouch type secondary battery including the same, wherein the inner layer includes a foaming activator capable of forming a protective gas layer.
Description of the Related Art
Technological developments and increased demands for mobile devices have led to a rapid increase in demands for secondary batteries as energy sources. Accordingly, researches for secondary batteries capable of meeting a variety of needs are emerging.
There are high demands for a prismatic type lithium secondary battery and a pouch type lithium secondary battery in terms of shape, which have a small thickness and are thus applicable to, for example, cell phones; and a lithium cobalt polymer secondary battery in terms of material, which has high energy density, discharge voltage, and stability.
The prismatic type lithium secondary battery is advantageous in protecting an electrode assembly against external impact and subjected to an easy injection process, but has a difficulty in reducing volume due to a fixed shape. Therefore, electric devices which use the prismatic type lithium secondary battery as a power source have a shortcoming in terms of a restricted design. Moreover, in terms of stability, the prismatic type lithium secondary battery has shortcomings in that gas or liquid is not smoothly vented and the risk of explosion thus increases due to accumulation of internal heat and gas, and ineffective release of internal heat results in cell degradation due to overheat in a short period of time.
The pouch type lithium secondary battery has advantages in that the pouch type lithium secondary battery is suitable for manufacturing a thin cell due to no restriction on its shape and size; it is easy to assemble the pouch type lithium secondary battery through heat fusion; and the pouch type lithium secondary battery has high stability due to easy vent of gas or liquid under the condition of abnormal behaviors. However, the pouch type secondary battery uses a thin soft laminated sheet (pouch) as a case thereof unlike the prismatic type, and thus has shortcomings in terms of low physical and mechanical strength, low sealing reliability, and low stability against external impact and the like.
Particularly, when large current flows into the pouch type secondary battery in a short time by local crush such as high temperature exposure, overcharge, an external short circuit, impurities in electrodes, and nail penetration, the electrode coated with an active material serves as a heat source and generates heat which thus rapidly increases temperature of the battery, so that reactions between an electrolytic solution and the electrode are accelerated and ignition of the battery is thus induced. In addition, an increase in internal pressure of the battery due to a gas generated by the reaction between the electrolytic solution and the electrode results in problems of swelling and explosion of the secondary battery. Such a risk of explosion may be the most critical drawback of lithium secondary batteries because the risk of explosion leads to a serious problem in stability.
Thus, an essential consideration in developing a pouch type secondary battery is to ensure its stability.
Using a pouch case having a high strength for stability ensures stability against external impact, but has a shortcoming in that it is difficult to form such a pouch case. In order to remedy this shortcoming, a method for improving the strength of a battery by using a soft pouch case and further forming a separate reinforcement layer on the surface thereof is suggested. However, in such a case, a separate reinforcement layer should be additionally provided, so that the size, volume, and manufacturing costs of the battery disadvantageously increase.